George R. Brubaker

Synthesis, spectroscopy and electrochemistry of some polynuclear complexes of aminoethanethiol☆

Abstract Polynuclear S-bridged complexes of the general formula {[Co 2 L 6 ]M} n+ with M = Co(III), Cd(II), Pb(II), Ni(II), Zn(II) and Hg(II) were prepared from [Co(2-aminoethanethiolate) 3 ] and the appropriate metal salt. Proton and 13 C NMR spectra are consistent with structures previously proposed for these species with 13 C chemical shifts dependent on the bridging metal ion. Electrochemical studies are consistent with a model in which an S-bonded ML 6 moiety ( i.e. , the bridging metal ion and the six aminoethanethiolate ligands) acts as a ‘dodecadentate’ ligand bonded to two Co 3+ ions. Reduction of the terminal cobalt ions in these trinuclear complexes is observed in the range −0.75 to −1 V vs. SCE on mercury, gold or glassy carbon working electrodes. For complexes with relatively labile bridging ions, the electrode reaction is irreversible, presumably due to rapid decomposition of the labile cobalt(II) product. For the tricobalt(III) derivative, however, the electrode reaction is reversible consistent with other recent observations on cage or otherwise stereorestrictive ligand systems [1].

Carbon 13 nuclear magnetic resonance spectroscopy of cobalt(III) complexes with flexible tetraamine ligands

The details of a systematic carbon 13 nuclear magnetic study of cobalt(III) complexes with the tetraamine ligands 1,8-diamino-3,6-diazaoctane (2,2,2-tet), 1,9-diamino-3,6-diazanonane (2,2,3-tet), 1,9-diamino-3,7-diazanonane (2,3,2-tet) and 1,10-diamino-4,7-diazadecane (3,2,3-tet) are presented. Carbon 13 NMR was found to be an extremely sensitive probe of the stereochemistry of this series of diamagnetic ‘Werner’ complexes. Our interpretation is based upon two parameters, the donation of electron density to the metal and the steric perturbation required for coordination. The stereochemistry of uns-cis(Co(2,2,3-tet)Val)++ is assigned based upon the carbon 13 NMR spectrum of the complex.

Models for metal-protein interaction: copper(II) complexes with some substituted cyclic dipeptides.

Abstract The substituted diketopiperazines (cyclic dipeptides) c-Gly-L-His, c-Gly-L-Asn, c-Gly-L-Gln, c-Gly-L-Asp, c-Gly-L-Glu, and c-Gly-L-Ser have been prepared and evaluated as models for metal ion interaction with protein side chains and amide linkages. These compounds, as protein models, have the advantage of completely eliminating terminal group effects. Copper complexes with substituted DKPs comply with all expectations derived from earlier studies of open-chain peptides, excepting the unusually high pH for Cu(II)-assisted deprotonation of the DKP ring amide nitrogen. Base hydrolysis of the DKP ring is inhibited by Cu(II); the L-Asn and L-Gln amide substituents are selectively cleaved in aqueous alkali to corresponding carboxylic acids, affording a simple model for metal-assisted L-Asparaginase and L-Glutaminase activity.

Quantitative estimates of steric effects: Intramolecular strain energy effects on the activation energy for aquation of some trans-dichlorotetraaminecobalt(III) complexes

Abstract In an effort to quantitatively estimate steric contributions to the aquation rates of a series of structurally related cobalt(III) tetraamine complexes, strain energy minimization calculations have been performed on the reactant and some plausible transition state structures. Free energies of activation Δ G * obs , are factored as: Δ G * obs , = Δ G * bb + Δ G * strain + Δ G * CF + Δ G * solvation + … where Δ G * bb is the free energy change associated with bond breaking, Δ G * solvation is the solvation free energy difference between the reactant and a proposed transition stare, Δ G * CF is the difference in crystal field stabilization between the reactant and a proposed transition state, and Δ G * strain is the strain energy difference between the reactant complex and a proposed transition state. The activation energy for the aquation of a hypothetical ‘strain free’ complex is defined as Δ G * int and reflects the energy required for the bond breaking step with all other terms. For the cations trans -( RR,SS )-dichloro-1,8- diamino-3,6-diazaoctanecobalt(III)( trans [Co(2,2,2- tet)Cl 2 ] + ), trans -(RR,SS)- or trans -( RS )-dichloro-1.9- diamino-3,7-diazanonanecobalt(III)(trans [Co(2,3,2- tet)Cl 2 ] + and trans -( RS )-dichloro-1,10-diamino-4,7- diazadecanecobalt(III)( trans [Co(3,2,3-tet)Cl 2 ] + ) Δ G * int is found to be a constant 123 kJ/mol. For the trans -dichlorocobalt(III) complexes with the ligands 1,4,7,10-tetraazacyclotridecane([13]-ane-N 4 ), 1,4,8, 11-tetraazacyclotetradecane([14]-ane-N 4 ), 1,4,8,12- tetraazacyclopentadecane([15]-ane-N 4 ) and 1,5,9,13- tetraazacyclohexadecane([16]-ane-N 4 ), Δ G * int lies in the range 133–139 kJ/mol.

OPTICAL ACTIVITY AND ELECTRONIC STRUCTURE OF SOME TRANS(DIACIDO)TETRAMINE COBALT(III) COMPLEXES

Abstract Electronic spectra for the trans(diacido)tetraminecobalt(III) complexes (anions may be glycinato, acetato R- or S.-alaninato, and the tetramine may be either 4,7-diaza-1,10-decanediamine (3,2,3-tet) or 4,7-diaza-5-methyl-1, 10-decanediamine (5-me-3,2,3-tet)) may be fit to a point charge model with D q xy =24.9 kK and D q z2kK; the pseudo tetragonal 1 A 1g – 1 A 2g and 1 A 1g – 1 E g transitions may therefore be assigned with some certainty. In the true lower symmetry of these complexes, the 1 E g state is split into two components. Regional rules predict a negative major CD component for trans(diacido)((–)-5-me-3,2,3-tet)cobalt(III) complexes while two components of opposite, but approximately equal rotatory strength have been observed. In this series of complexes, optical activity may be induced along a ‘tetragonal’ axis, within a ‘tetragonal’ plane, or both. The 1 A 1g–1 A 2g transition shows no rotatory strength for optically active centers on the z axis. Of the two bands observed in the regio...

Cobalt(III) complexes with cyanide and the tetraamines 1,9-diamino-3,7-diazanonane and 1,10-diamino-4,7-diazadecane

Abstract The reaction of trans -(dichloro)(1,9-diamino-3,7-diazanonane)cobalt(III) chloride and trans -(dichloro)(1,10-diamino-4,7-diazadecane)cobalt(III) chloride with sodium cyanide in aqueous solution is found to yield products with either trans or uns- cis stereochemistry depending on the reaction conditions. The stereochemistry of the products was determined by proton and carbon-13 NMR, electronic and vibrational spectroscopy. This is the first case where uns- cis diacido complexes with cobalt(III) and the tetraamines 1,9-diamino-3,7-diazanonane and 1,10-diamino-4,7-diazadecane have been prepared directly. Previous preparations of uns- cis complexes with these tetraamines have required an uns- cis complex with a chelating ligand as an intermediate. The stability of the uns- cis complexes is attributed to a greater degree of π bonding in the uns- cis complex compared with the corresponding trans complex. The difference in π bond energy must be greater than the 6 Kcal mol −1 difference in strain energy estimated from a molecular mechanics calculation.

Computer Enhanced Chemical Education: Any Computer Can be Used

During my formative years as an Assistant Professor of Chemistry, I had heard a great deal about computers in Chemistry. Under the influence of Peter Lykos and Audrey Companion, OCPE, CONDUIT, NCECS, and later PLATO, became familiar names; I had also heard of Harrison Shull, Joe Lagowski, Al Lata, Ron Crain, Ron Collins and Stan Smith, among others. Yet, except for occasional contact at a professional meeting, I did not know these men, and I had not seen, first hand, examples of their work.